Process of refining cellulose pulp



pulp having a hig Patented Dec. 9, 1930 UNITED STATES PATENT OFFICE 1 enonen a. Bream-am) m'ron 0. mm, or 3mm, NEW mrsnmn, assrenoas T0 snows or 13mm, msw mnrsmn, a conrona'rron' or 1mm PROCESS OF REFINING CELLULOSE PULP Io Drawing.

This invention hasfor its object the treat-' ment of cellulose pulp produced by a digestion of raw cellulosic material, and more especially sulphite pulp, to produce a refined color and a great proportion of resistant or alpha cellulose and to accomplish this result with a highyield and with a small proportion of reagents.

Raw sulphite pulp is characterized by a relatively low pentosan content as compared 1 largely removed in the soda or sulphate with that of a pulp initially produced by an alkaline digestion, but it contains relatively larger proportions of less-resistant celluloses, such as beta and gamma celluloses, which are process.

The present invention, in addition to the objects first mentioned, has also for its .ob-

jects to produce a product which is low in pentosan content and, while low in potash solubility, nevertheless has a high percentage of alpha cellulose, so that it is available not only for the production of cellulose esters, e. g,, nitrocellulose or cellulose xanthate, but also for the production of a paper which has a marked similarity to'paper produced from cotton rags, in its strength, softness, folding endurance, and resistance to tear.

In carrying out our process three operations are performed, the first of which is a treatment of the pulp with a relativel small proportion of an oxidizing agent for t e purpose 6f removing or dissolving some of the resinous or li'gneous components of the pulp and of rendering some of the more refractory components more readily soluble in the alkaline treatment to be described. This treat-- ment is in no sense a bleaching operation, as

it does not have the effect of whitenin the pulp. The oxidizing reagent reacts wit the resins and the more susceptible li gneous con tent of the pulp, and by the elimination there of increases the proportion of the resistant or alpha cellulose in the total pulp. Any suitable oxidizing agent may be employed for this purpose, such as calcium hypochlorite or chlorine,'but care should be taken to emplo only a small proportion of the reagent such as will not react appreciably withthe cellulose fibers. After this operation is per- Application fled Kay 87, 195M.v Serial formed, the resulting productis preferably washed to free it from the reaction products. The second step of our process consists in subjecting the pulp to an alkaline liquorwith the application of heat until the alkali hasdissolved the greater portion of the non-resistant (beta or gamma) cellulose, and has dissolved the residual reaction products resulting from the first reaction as well as other resinous or ligneous matter which may remain inthe pulp. As a result of this operation, the proportion of resistant cellulose to the total quantit oi pulp is materially increased. The pulp is now washed thorou hly to remove all of the alkali and the reaction products, and is then subjected to the third has been concluded, the pulp is now washed again thoroughly and treated with antichlore We have found that, by following a process such as herein described, it is possible to effect marked economies in the cost of the reagents employed and at the same time to secure a high yield based on the initial unbleached pulp, and to produce a pulp having a maximum proportion of resistant cellulose. As illustrating more in detail how the process may be carried on in connection with sulphite pulp produced from spruce, the characteristics of which depend more or less upon the varying factors of the fiber-liberating process by which it is produced, it may be assumed, for example, that the unbleached pulp has a total cellulose content of, say, 96.8%, about resistant or alpha cellulose, with a copper number ofapprox'imately 3.5. A

10% stock suspension of this pulp is now thereto from 1% to 2% of bleach powder in solution based on the weight of the fiber and on calcium hypochlorite powder containing 35% available chlorine. In lieu thereof we may employ from 0.3% to 0.6% chlorine based on the pulp. The mass is subjected .placed in an openvat, and there is added ment than before.

to agitation for about thirty minutes. As previously stated, the oxidizing agent reacts with the more susceptible ligneous content of the pulp without aflecting any material or perceptible bleaching operation upon the pulp. Infact, the pulp appears to be of a darker color when subjected to this treat- As a resultlof this step of the process, experience has shown that theproduct after washing contains about 97.2% total cellulose, about 87% to 88% resistant or alpha cellulose, and that the copper number is reduced to about 3. The pulp is now transferred-to an open vat and there is added thereto an alkaline liquor of a strength of.0.1% to 0.5% caustic soda or from 1% to 5% caustic soda based on dry pulp. For this purpose We may use as the alkaline reagent either caustic soda or caustic potash or a sodium carbonate or other alkaline solution. The pulp is digested preferably at atmospheric pressure but with the application of sufiicient heat to efi'ect the reaction .the residual undissolved reaction products reslting from the first reaction, and such other resinous or ligneous matter as was not affected by the treatment with the first oxidizing agent. At the end of the period of digestion, the pulp is now thoroughly washed for the removal of all of the reaction products, and experience has shown that at this stage the mass contains about 99.3% total cellulose, about 93% to 94% resistant cellulose, and has a copper number of about 2. The material is n0w ready for the bleaching action, by which practically all of the coloring matter is removed and the pulp is bleached to the desired degree of whiteness. In

this case the pulp is subjected to the bleaching action of the equivalent of from 2% to 4% of 35% bleach based on the pulpcalcium hypochloite being preferably employed 1 for this purpose. As a practical matter, the

stock may be in the form of a 10% suspension and the operation may be carried on at a temperature of about 80 F. for from, say, six to eight hours. The pulp is then thoroughly washed and treated with calcium bisulphite, or other suitableantichlor, to neutralize any bleach that may remain in the product. At thispoint we have found that the stock contains about 99.6% total cellulose, about 92% to 93% resistant or alpha cellulose, and that the stock exhibits a copper number of about 2.3. Instead of carrying on the third step with hypochlorite alone, we may combine with the hypochlorite a certain proportion of caustic soda or sodium carbonate, which has the effect of preventing localized overbleaching. and retards the 'lulose esters.

bleaching operation, as pointed out in our application Serial No. 716,153 filed concurrently herewith, and issued as Patent No. 1,639,704, on August 23, 1927. If the reagent for the bleaching operation consists of a mixture of calcium ,hypochlorite and caustic soda, the final product will contain approximately 99.6% totalcellulose, 93% to 94% alpha cellulose, with a copper number of about 1.7. If "c:arrying"on the process herein described,'.-there -is a much smaller consumptionofbleach with a'higher pulp yield having a higher percent. of resistant cellulose than has heretofore been possible with the ordinary bleaching operations. In fact, although the 'total cellulose yield based on the initial unbleached pulp is somewhat lowerutfrom 10% to 15%) than with the usual bleaching operation, there is only about one-half ,of the amount of bleach used in carrying out our process than is employed in the usual bleaching operation. There is produced as'a result of our process, however, a

cellulose product which lends itself for the production of cellulose nitrate and other cel- The average fiber length of the stock is nearly double that of stock produced by the ordinary bleaching methods. A paper produced from the pulp is much softer and at the same time possesses a greater tearing resistance than the paper made from ordinary bleached p'ulp'. It will be seen that cellulose pulp such as We have herein described as resulting from the practice of our process is comparable in value with cotton in the production of cellulose esters, cotton having ordinarily a copper number of, say, 1.5 and com prising, say, from 94% to 96% alpha cellulose, but has a great advantage over'cotton in point of economy.

While in the example we have given as illustrating how the process may be practised we have indicated certain values which we have obtained and have also stated specific proportions of the several reagents, we would not consider it a departure from the scope of this invention if these proportions were somewhat changed, assuming that comparable results are secured. In carrying on experimental Work in connection with this process, we have noted that when unbleached chemical wood pulp is subjectedto the action of an oxidizing agent, with a given proportion of the reagent during a givenperiod of time, the first consumption of the oxidizing agent is accompanied by a rise in the proportion of alpha cellulose in the pulp, but that a critical pointis reached so that the further consumption of the oxidizing agent is accompanied by a marked decrease in the proportion of the alpha or resistant cellulose con-- tent of the pulp. We have also'noted that the copper number decreases with the first consumption, of the oxidizing agent up to the critical point and thereafter rises. Consequently, in carrying on the first step of our process, we employ a relatively small amount of oxidizing.agent,that is, based on the amount of pulp,and terminate the operation when or before this critical point is reached, this taking. about thirty minutes under the conditions stated in the specific example. In the second step of the process,

however, the reaction with the alkaline liquor is carried on until the less resistant cellulose is dissolved and removed without effecting a marked deterioration in the alpha cellulose. This may be favored by adding the total quantity of alkali fromitime to time in relativelysmall proportion so as to prevent an overconcentration. In the bleaching operation, however, it may be stated that this is carried on until the desired color has been achieved by the dissolving and removal of the coloring matter, and here again care should be exerthe most desirable paper-making characteristics.

2. A rocess which comprises treating unbleache wood pulpwith an oxidizing agent until the residue from this treatment shows a maximum alpha cellulosecontent, and then digestin this high alpha-cellulose-containing resi ue in an alkaline liquor under conditions to increase further the alpha cellulose content of the residual pulp, thus preserving or developing in the fibres the most desirable paper-making characteristics, and'bleaching the digested pulp.

3. A process which comprises treating unbleached chemical pulp with an'oxidizing agent until the residue from this treatment shows a maximum alpha cellulose content,

washing the residue, digesting the washedresidue in an alkaline liquor under atmospheric pressure under conditions to increase further the alpha cellulose content of the residual pulp, thus preserving or developing in the fibres the most desirable paper-making characteristics, again washing the digested pul and bleaching the washed pulp in a hypoc lorite bleach liquor containing sufiicient free alkali to maintain a condition of distinct alkalinity throughout the bleaching operation.

In testimony whereof we have affixed our signatures.

GEORGE A. RICHTER. MILTON O. SCHUR.

gesting the pulp at atmospheric pressure with,

eat, we secure certain advantageous results and marked economy of operation. That is to say, this makes ossible a continuous operation in open vesse s, thus ensuring simplicity of operation and ease of control. Again, we secure an economy in the use of steam in heating, and maintain the heat of the alkaline o stock suspension; a greater yieldof product based on original raw sulphite pulp is secured than where the pressure digestion is em- -ployed and, owing to the fact that in digesting un er atmospheric pressure the temperature to which t of the alpha cellulose content and yet secure the selective reaction of the less-resistant celluloses and thereby produce a product which when made into paper has a greater stren w h, resistance to tear, and foldin endurance an one which has been digested under pressure.

What we claim is:

1. A process which comprises treatin un:

bleached, previously-liberated ulp wit an oxidizing agent until the resi ue from this treatment shows a maximum alpha cellulose content, and then treating this high alphacellulose-containing residue with an alkaline liquor under conditions to increase further the alpha cellulose content of the residual ulp,

e mass is heated is consequently limited, we prevent the degradation thus preserving or developing in the bres v 

